Steel sheet and plated steel sheet

A steel sheet having a chemical composition comprising, by mass %,

• C:
• 0.05 to 0.40%,
• Si:
• 0.2 to 3.0%,
• Mn:
• 0.1 to 5.0%,
• sol. Al:
• 0.4 to 1.50%,
• P:
• 0.0300% or less,
• S:
• 0.0300% or less,
• N:
• 0.0100% or less,
• B:
• 0 to 0.010%,
• Ti:
• 0 to 0.150%,
• Nb:
• 0 to 0.150%,
• V:
• 0 to 0.150%,
• Cr:
• 0 to 2.00%,
• Ni:
• 0 to 2.00%,
• Cu:
• 0 to 2.00%,
• Mo:
• 0 to 1.00%,
• W:
• 0 to 1.00%,
• Ca:
• 0 to 0.100%,
• Mg:
• 0 to 0.100%,
• Zr:
• 0 to 0.100%,
• Hf:
• 0 to 0.100%, and
• REM:
• 0 to 0.100% and
• having a balance of Fe and impurities,
• wherein:
• an internal oxidation layer including fine granular oxides, the fine granular oxides having a grain size of 20 nm or more and 100 nm or less, and coarse granular oxides, the coarse granular oxides having a grain size of 150 nm or more and 600 nm or less, is present in a surface layer of the steel sheet, and,
• in a cross-section of the surface layer of the steel sheet,
• a number density of the fine granular oxides in the internal oxidation layer is 4.0/μm2 or more,
• a number density of the coarse granular oxides in the internal oxidation layer is 4.0/25 μm2 or more and 30.0/25 μm2 or less, and
• a surface depleted layer with a steel composition not including the fine granular oxides and coarse granular oxides which satisfies, by mass %, Si≤0.6% and Al≥0.05% is included at a depth of ½ of the average depth of the internal oxidation layer.